A gas turbine engine fan or compressor is typically formed with one or more stages including a disk from which extends radially outwardly a plurality of circumferentially spaced apart rotor blades. Each rotor blade includes an airfoil and a dovetail at its root, with the dovetail being radially retained in a complementary slot in the perimeter of the disk. During operation, the disk and blades attached thereto rotate, with the blades developing substantial centrifugal force which is carried downwardly through the respective dovetails and into the disk. The dovetails must be suitably configured and sized for supporting the blades with a suitably low level of stress for obtaining a useful life in operation.
In some gas turbine engine designs, the conventionally known radius ratio and blade solidity are such that the blades are disposed relatively close together around the perimeter of the disk, with the disk being relatively small in diameter compared to the airfoils which results in the inability of conventional dovetail designs to carry centrifugal loading at suitable levels of stress for enjoying a useful service life. Accordingly, the airfoils are manufactured integrally with the disk in a one piece component conventionally known as a blisk. A blisk is typically manufactured from a one piece solid forging which is conventionally machined using either a mill or electrochemical machining (ECM) electrodes. With the blades being integral with the disk, satisfactory levels of stress may be obtained in the blisk during operation for obtaining a useful life.
In yet another application of a blisk, a continuous mid-span or part-span shroud is provided to bifurcate the airfoils into inner, or hub, airfoils and outer, or tip, airfoils so that airflow may be channeled separately thereover in different inner and outer flowpaths for use in a variable cycle gas turbine engine for example. The 360.degree. continuous shroud not only seals or prevents radial crossflow or leakage between the inner and outer flowpaths but substantially increases the overall stiffness of the blisk to raise its vibrational frequencies into a more desirable range. The additional mass provided by the shroud itself also generates centrifugal loads during operation which in part are carried through hoop stresses generated in the shroud during operation. Some of the shroud centrifugal loads however are also carried through the inner airfoils to the disk.
The continuous shroud in a blisk poses numerous manufacturing problems since the shroud itself significantly limits access to the region of the inner airfoils and inner diameter of the shroud itself which are formed by removal of material from the original solid forging. Furthermore, normal wear or damage to the shroud during operation of the engine will require a more complex process in order to remove any damaged portion of the shroud and suitably replace it. The damaged portion must be cut from the shroud, with a replacement portion being welded thereto.